A Million Years Without Oxygen in the Seas Caused the First Great Mass Extinction

The first of the five great mass extinctions of species on Earth occurred around 450 million years ago (in the late Ordovician) because the oceans ran out of oxygen for at least a million years, which triggered the annihilation 85% of marine life when then most of the species inhabited the seas.

This is confirmed by six scientists from universities in Canada, United States and the United Kingdom in an article published in the journal ‘Proceedings of the National Academy of Sciences’.

Over the years, scientists have discovered that among the main causes of mass extinctions are massive volcanic eruptions, global warming, asteroid collisions and acid oceans. Other factors are methane eruptions and marine anoxic episodes, that is when the oceans lose vital oxygen.

What triggered the mass extinction of marine animals and plants in the Late Ordovician has remained a mystery until now. That period was a dynamic time interval in the history of the Earth that recorded a significant increase in marine biological diversity and thermal cooling. So far, researchers have suggested that that temperature drop led to massive annihilation 450 million years ago.

ANOXIA ABRUPTA

The new study mentions an abrupt anoxia in the oceans of the planet as the main cause of the first great mass extinction on Earth, after deciphering the geochemical evidence left in a sediment of marine limestone in Quebec (Canada).

“This extinction is the first of the five great extinctions that hit the Earth and our research indicates that it coincided with the abrupt development of generalized oceanic anoxia that lasted at least one million years,” says Maya Elrick of the University of New Mexico. (U.S).

Elrick notes that anoxia was driven by global cooling, which reorganized ocean circulation on a large scale and reduced oxygenation of deep oceans and improved nutrient flows, which triggered the proliferation of phytoplankton and expanded the areas of low concentrations of oxygen.

The team compared the conditions 450 million years ago with those of today and determined that there was an increase of around 15% of anoxic seafloor during the massive extinction of the Late Ordovician. The modern ocean has less than 0.5% of the marine bottom without oxygen (mainly, the Black Sea).